With the spread of IP networks, a telephone service using IP (VOIP) is becoming widespread, which is expected to apply to a TV conference and a voice conference. Because conversation is made among a multiplicity of persons in the TV conference and the voice conference, generally, a hands-free communication function enabling conversation with hands free is essential. However, there arises a problem of acoustic echo such that a sound regenerated by a speaker is recorded through a microphone and undesirably heard by the opposite party.
FIG. 1 is a diagram illustrating hands-free speech communication. In the speech communication between an opposite terminal 10, which is a hands-free speech communication device, and a self-terminal 20, the voice recorded through a microphone 11 of the opposite terminal 10 is regenerated and output from a speaker 22 of self-terminal 20, and the voice recorded through a microphone 21 of self-terminal 20 is regenerated and output from a speaker 12 of the opposite terminal 10. At this time, there arises the problem of an acoustic echo, in which the voice being output, for example, from speaker 22 of self-terminal 20 is recorded through microphone 21, and undesirably output from speaker 12 of the opposite terminal 10. For this reason, each speech communication device (terminal 10, 20) has an incorporated echo canceller 23 for preventing such the acoustic echo. Here, in FIG. 1, the illustration of an echo canceller in the opposite terminal 10 is omitted.
FIG. 2 is a diagram illustrating the principle of the echo canceller. The echo canceller examines the correlation of the frequency spectra between a speaker signal (reference signal) and a microphone signal including an echo for each frequency band, and when the correlation is large, the echo is decided to be large, so that an echo suppression amount is increased. To the contrary, when the correlation is small, the echo is decided to be small, so that the echo suppression amount is decreased. The above process is performed for the entire frequency bands, and according to the magnitude of the correlation, the echo is suppressed.    Patent document 1: Japanese Patent Application Laid-open No. Hei-11-331046.    Patent document 2: Japanese Published Patent Application No. 2003-517782.